"He finally signed the plan on technical preparations of strategic rockets, ordering them to be on standby to fire so that they may strike any time the U.S. mainland, its military bases in the operational theaters in the Pacific, including Hawaii and Guam, and those in South Korea," KCNA said.

The announcement came a day after two U.S. B-2 Spirit stealth bombers flew non-stop practice missions from their base in Missouri to the Korean Peninsula, a distance of 6,500 miles, and back in a show of U.S. commitment to its Asia-Pacific allies.

North Korean media quoted Kim as saying the stealth bomber practice mission was a U.S. ultimatum to ignite a nuclear war at any cost and said Kim "declared the revolutionary armed forces of the DPRK [Democratic People's Republic of Korea, North Korea's official name] would react to the U.S. nuclear blackmail with merciless nuclear attack, and war of aggression with an all-out war of justice."

During a bombing drill over the Korean Peninsula, the U.S. B-2 stealth bombers, with their radar-evading capability, dropped "inert munitions on the Jik [islet] Do Range" and returned "to the continental U.S. in a single, continuous mission," said a statement carried on the website of the U.S. Forces in Korea.

A South Korean official told Yonhap that while the B-2 stealth bombing exercise was a routine part of the current U.S.-South Korean military drills, "we take First Chairman Kim's order as a step to respond to this [exercise]."

Passage of the U.N. Security Council resolution imposing further sanctions on North Korea is "a pretty clear indication that there is a unanimity of opinion across the globe about the need for North Korea to live up to their international obligations," Earnest said.

The Security Council resolution is also an indication that the United States is working with not just its allies but also Russia and China in seeking a peaceful solution, he said

"So the path to peace for the North Koreans is pretty clear," Earnest said. "They need to end the provocative acts and the bellicose rhetoric. They need to abandon their nuclear program. They need to live up to their international obligations. And upon doing so, they will be welcomed back into the international community."

At the U.S. State Department, spokeswoman Victoria Nuland said, "When a country says the kinds of things that the DPRK is saying, you have to take it seriously and you have to take steps to ensure that when we say in response we can and will defend our own nation and we can and will defend our allies, that that is credible."

Nuland said North Korea is "causing us to have to ensure that our defenses are appropriate and strong both for ourselves and for our allies."

Nuland, quoting President Barack Obama, said North Korea can come out of its isolation if it is willing to meet its international obligations and fulfill its denuclearization and other commitments.

"But in the meantime, we're going to do what we need to do to defend ourselves and our allies," she said.

Earlier, a U.S. official told CNN North Korea "is not a paper tiger" and it would not be "smart to dismiss its provocative behavior as pure bluster."

Pentagon spokesman George Little told CNN it is important to remain calm because no one "wants there to be war on the Korean Peninsula."

Analysts have said despite the latest threat North Korea is nowhere close to being technologically capable of delivering a nuclear warhead on a missile.

Little told CNN the United States in any case is watching North Korea's missile capabilities.

"The important thing is for us to stay out ahead of what we think the North Korean threat is, especially from their missile program," he said. "They've been testing more missiles, and they've been growing their capabilities and we have to stay out ahead."

The isolated, impoverished Communist country's anger has reached a feverish pitch since the U.N. Security Council tightened its sanctions following its December long-range missile test and its February nuclear test, its third since 2006, observers said.

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